1. Field of the Invention
The present invention relates to a apparatus and method for receiving and demodulating data modulated in a pseudo-ternary form, and more specifically to a device and method for receiving and recovering data modulated in a pseudo-ternary form and transferred over a pair of signal lines in which a transformer is inserted.
2. Description of the Related Art
In recent years, various portable computers have been developed. Some of the portable computers are designed so that they can be attached to an expansion unit in order to increase their functions. The bus of a portable computer is connected with the bus of an expansion unit. This bus connection allows devices on the bus within the expansion unit to be handled in the same way as those within the portable computer.
Many computers use a PCI (Peripheral Component Interconnect) bus. The bus connection between the computer and the expansion unit is implemented by physically connecting the PCI buses of the computer and the expansion unit together through docking connectors that are installed in the computer and the expansion unit. Each docking connector has as many pins as there are signal lines in the PCI bus.
According to such a configuration, however, both the computer and the expansion unit need a large amount of space to house the docking connector. This involves difficulties when a computer is to be downsized. In addition, the position in the computer where the docking connector is mounted needs to coincide with that of the docking connector of the expansion unit, which imposes restrictions on the physical structure of the casing of the computer in developing products.
From such a background, the demand has increased for developing a technique to connect two buses through a high-speed serial interface. The use of the high-speed serial interface allows the computer and the expansion unit to be connected together through a thin, flexible serial cable. Expected serial data transfer methods suitable for implementation of the high-speed serial interface include a method of utilizing a difference signal, and one of utilizing a bipolar signal.
The former carries out serial data transfer through the use of paired signals which are reversed in phase with respect to each other. Binary data at logic level 1 or 0 is transmitted in the form of paired signals which are reversed in phase with respect to each other. The latter makes transfer of serial data through the use of three voltage levels: +V, 0, and −V. Binary data at logic level 1 or 0 is transferred in the form of either +V, 0, or −V.
In order to implement a high-speed serial data transfer, some provision must be made for increasing noise immunity. The immunity of signals to be transferred to noise can be improved by inserting a transformer in a serial transmission line. Since the insertion of a transformer allows DC components of transmitted signals to be cut, the immunity to extraneous common mode noise, such as static electricity, can be improved. However, for signal transmissions over a transformer, attention must be paid to the following two points:
1) In order to prevent magnetic saturation of the transformer, it is required to prevent current from continuing to flow in one direction.
2) The total time during which a voltage is applied in the positive direction to the transformer and the total time during which voltage is applied in the negative direction to the transformer should be made substantially equal to each other. In the event that the time during which voltage is applied in one direction to the transformer is longer, there arises a problem that the voltage waveform of a signal produced on the secondary side of the transformer shifts in the positive or negative direction.
To meet the two requirements, it is best to modulate data in a pseudo-ternary form for transmission. In the pseudo-ternary signal transmission, which is one of the bipolar-signal-based methods, data modulated in a pseudo-ternary form is transferred over a transmission line in which a transformer is inserted. The pseudo-ternary transmission uses three values: a positive value (positive pulse), a negative value (negative pulse), and zero (no pulse). Each binary data to be transmitted is converted into one of three states of a positive pulse, a negative pulse and no pulse. The positive pulse state is the state in which current flows in the positive direction in the transformer. The negative pulse state is the state in which current flows in the negative direction in the transformer. The no pulse state is the state where no current flows in the transformer.
When such a pseudo-ternary signal transmission technique is used, the possible upper limit of the data transmission rate depends on the performance of the receiving device for reception and demodulation of the data. As the data transmission rate increases, it becomes more difficult to receive and demodulate correctly the three values of +V, −V and ±0 (corresponding to a positive pulse, a negative pulse, and no pulse, respectively) that will appear on the secondary side of the transformer. In other words, it is easy for errors to occur in data demodulation.